The correct answer is inattentional blindness
People often do not notice major changes in their visual field. And this is easily noticed when you are focused on something else - like counting the number of passes on a basketball.
The implication of this for communication / design / marketing is that the fact that people look at something does not mean that they are paying attention. Just because it is there, in their face, does not mean that they are really seeing (perception is the vision of the brain). If you want people to notice the changes, you may need to be very direct about showing what the changes are and where they are.